©  Copyright 2013 Natural gas, as it is used by consumers, is much different  from the natural gas that is brought from underground up to  the wellhead. Although the processing of natural gas is in  many respects less complicated than the processing and  refining of crude oil, it is equally as necessary before its use  by end users.  The natural gas used by consumers is composed almost  entirely of methane. However, natural gas found at the wellhead, although still composed primarily  of methane, is by no means as pure. Raw natural gas comes from three types of wells: oil wells,  gas wells, and condensate wells. Natural gas that comes from oil wells is typically termed  'associated gas'. This gas can exist separate from oil in the formation (free gas), or dissolved in  the crude oil (dissolved gas). Natural gas from gas and condensate wells, in which there is little or  no crude oil, is termed 'nonassociated gas'. Gas wells typically produce raw natural gas by itself,  while condensate wells produce free natural gas along with a semi-liquid hydrocarbon  condensate. Whatever the source of the natural gas, once separated from crude oil (if present) it  commonly exists in mixtures with other hydrocarbons; principally ethane, propane, butane, and  pentanes. In addition, raw natural gas contains water vapor, hydrogen sulfide (H2S), carbon  dioxide, helium, nitrogen, and other compounds.  Natural gas processing consists of separating all of the various hydrocarbons and fluids from the  pure natural gas, to produce what is known as 'pipeline quality' dry natural gas. Major  transportation pipelines usually impose restrictions on the make-up of the natural gas that is  allowed into the pipeline. That means that before the natural gas can be transported it must be  purified. While the ethane, propane, butane, and pentanes must be removed from natural gas,  this does not mean that they are all 'waste products'.    Chemical Engineering & Gas Processing